1. Field of the Invention
The present invention generally relates to communication systems, and more particularly, to a system and method for detecting particular tones in communication systems using the discrete multi-tone standard.
2. Discussion of the Related Art
In recent years, telephone communication systems have expanded from traditional plain old telephone system (POTS) communications to include high-speed data communications as well. As is known, POTS communications include the transmission of voice information, control signals, PSTN (public switched telephone network) information, as well as, information from ancillary equipment in analog form (i.e. computer modems and facsimile machines) that is transmitted in the POTS bandwidth.
Prompted largely by the desire of large businesses to reliably transfer information over a broadband network, telecommunications service providers have employed discrete multi-tone, hereinafter DMT, systems to provide a plethora of interactive multi-media digital signals over the same existing POTS twisted-pair lines. The provision of asynchronous digital subscriber lines (ADSL) using DMT systems to customer premises has proliferated over recent years. Since ADSL signals are transmitted in a higher frequency band than that of the POTS frequency band, transmitting signals from both the POTS and ADSL frequency bands over the same twisted-pair telephone line (even at the same time), generally is not a problem. Specifically, the POTS frequency band is generally defined from 0 Hz to 4 kHz, while ADSL frequency bands are generally defined by a lower cutoff frequency of approximately 26 kHz, and an upper cutoff frequency of approximately 1 MHz.
In the past, a combination of circuits termed hybrids, and POTS splitters have served to buffer ADSL equipment from distortions and interference introduced in the ADSL frequency bands from the lower frequency POTS equipment. In a DMT-G.Lite standard configuration, the POTS splitter is no longer present. As a result, POTS equipment operates on the same twisted-pair phone line that is being used to deliver ADSL services. POTS equipment operating in this configuration is subject to interference from low frequency harmonics generated within the ADSL equipment.
Conversely, and of greater significance, the presence of abrupt changes in line conditions due to ringing, customer premises noise, POTS handset pick-up, and on/off-hook transitions from ancillary equipment, can disrupt ADSL transmissions. Splitterless operation of an ADSL often incurs a significant and abrupt insertion-loss change upon the off-hook terminating impedance change of the POTS device. DMT systems, by nature of their distribution across multiple frequency bands, are capable of retuning devices to optimize data transfer for changing line conditions. DMT devices selectively transfer bits from the data stream in those discrete frequency bands that are uncorrupted from amplitude modulation radio interference and unaffected by phone system bridge taps, thereby tuning, or maximizing performance under changing line conditions.
Tuning of DMT system parameters is currently performed in two distinct ways: initial training, hereinafter called xe2x80x9cfull retrain,xe2x80x9d and bit loading/swapping, an online optimization procedure. Another often suggested means to retune a system is a fast retrain of the connection. A full retrain of the system connection results in a temporary loss of service and is undesirable under most conditions. Of the methods used to tune DMT parameters, fast retrain is best suited to overcome transient effects, while bit loading/swapping is more adapted to slowly varying changes. The fast retrain method is more robust than bit loading/swapping and provides for a more optimized system since it can actively readapt other system components such as equalizers and echo-cancelers to the system noise environment.
The fast retrain algorithm is triggered when either the central office or the remote transmission unit sense the need to transition from the current parameter profile to a more appropriate previously stored parameter profile. The most typical situation that triggers a fast retrain is when a POTS device goes on/off hook. These transitions create impedance transients that adversely affect the ADSL frequency spectra. When the central office""s equipment initiates a fast retrain procedure, the transmission device discontinues sending data with DMT cyclic prefix and begins sending two signal tones (DMT signal tones 64 and 68) without cyclic prefix. The remote device within the customer""s premises, which is still configured to accept data with cyclic prefix, has to detect the transmission of the two signal tones to trigger the fast retrain event.
Prior art tonal state detectors generally use a measure of total signal power within the frequency band to determine if a particular tone is being used for data transfer. Specifically, prior art detectors observe the total power within the frequency band of tone 68 and compare it to a preset threshold or to the energy of adjacent tones in order to make the determination that the central office is sending a control signal. The former method has the. drawback of requiring a decision value based on unknown parameters. The threshold value will depend upon ambient noise, receiver gain, loop attenuation, and other environmental factors. The latter method detects sudden changes in a power ratio equal to the power of tone 68 divided by the average power of adjacent tones. The method of comparing tone 68 power to power levels in adjacent tones is more complex in that it requires increased computation time to determine average powers in several tones. In addition, DMT schemes by their very nature can vary power within each of the tones by shifting bits among the individual frequency bands, making it impossible to anticipate a power ratio that will correspond to a control signal in all cases. Lastly, the latter method requires a division step that is not easily implemented on digital signal processors.
Accordingly, it is desired to provide a system and method that efficiently, accurately, and quickly detects the condition in a DMT system where either the central office or a remote device is requesting a fast retrain of the system.
Certain objects, advantages and novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the objects and advantages of the present invention, the present invention is directed to a system and a method for detecting a condition where either the central office or a remote device in a DMT system is requesting a fast retrain of the system. The innovative solution of the present invention focuses on the characteristics of tone 68 in making the determination to trigger a fast retrain of the DMT system. More specifically, the present invention evaluates and identifies a phase shift in tone 68 to identify when either party is requesting a fast retrain. It has been determined that when a DMT system is in data transfer mode that the phase shift on tone 68 will be random in nature. When the central office has requested a fast retrain of the system by removing the cyclic header on each frame of data being transmitted on tone 68, the phase of tone 68 will have a constant shift of xcfx80/2 at each symbol. The following relationships explain the phase shift at each symbol:
There are 68 periods for tone 68 in one symbol.
The cyclic prefix represents 16/256 of a symbol and covers (16/256)*68=4.25 periods.
4.25 periods modulo 2xcfx80 yields 0.25 periods or a xcfx80/2 phase shift due to the removal of the cyclic prefix.
In accordance with one embodiment of the present invention, a system and method is provided for detecting a constant phase shift on tone 68 by estimating the mean phase shift over a predetermined number of samples and determining whether it exceeds a predetermined threshold. In an alternative embodiment, a system and method is provided for detecting a constant phase shift on tone 68 by calculating the cross product of the real and imaginary portions of successive frequency domain values of tone 68. The result of the cross product will approach 0 if tone 68 is in data transfer mode. The result in the mean approaches the square of the average energy in the tone if the cyclic prefix has been removed and tone 68 is indicating a fast retrain of the system is in order.